Testing glow plugs
Time and again there are discussions in forums about removing a glow plug and connecting it to an electrical source to test whether it still glows. Such "testing methods" can cause serious injuries due to burns!
Also, incorrect installation of glow plugs can damage both the glow plug itself and the engine.
Therefore, testing and replacing glow plugs should always be left to the experts in the automobile repair shop. Only they have the necessary knowledge and the right equipment.